Stress Components Research Articles

On the use of the Astarita flow field for viscoelastic fluids to develop a generalised Newtonian fluid model incorporating flow type (GNFFTy)

The two-dimensional, steady, homogeneous flow field proposed by Astarita (J. Rheol., vol. 35, 1991, pp. 687–689) is studied for a range of viscoelastic constitutive equations of differential form including the models due to Oldroyd (the upper and lower convected Maxwell; UCM/LCM), Phan-Thien and Tanner (simplified, linear form; sPTT) and Giesekus. As the flow is steady and homogeneous, the sPTT model results also give the FENE-P model solutions via a simple transformation of parameters. The flow field has the interesting feature that a scalar parameter may be used to vary the flow ‘type’ continuously from solid-body rotation to simple shearing to planar extension whilst the rate of deformation tensor $\boldsymbol{\mathsf{D}}$ remains constant (i.e. independent of flow type). The response of the models is probed in order to determine how a scalar ‘viscosity’ function may be rigorously constructed which includes flow-type dependence. We show that for most of these models – the Giesekus being the exception – the first and second invariants of the resulting extra stress tensor are linearly related, and for models based on the upper convected derivative, this link is simply via a material property, i.e. half the modulus. By defining a frame-invariant coordinate system with respect to the eigenvectors of $\boldsymbol{\mathsf{D}}$ , we associate a ‘viscosity’ for each of the flows to a deviatoric stress component and show how this quantity varies with the flow-type parameter. For elliptical motions, rate thinning is always observed and all models give essentially the UCM response. For strong flows, i.e. flow types containing at least some extension, thickening occurs and only a small element of extension is required to remove any shear thinning inherent in the model (e.g. as occurs in steady simple shearing for the sPTT/Giesekus models). Finally, a functional form of a viscosity equation which could incorporate flow type, but be otherwise inelastic, the so-called GNFFTy (generalised Newtonian fluid model incorporating flow type, pronounced ‘nifty’), is proposed. In the frame-invariant coordinate system proposed, this model is also capable of capturing normal-stress differences.

Effects of 12 weeks of functional and resistance training on stress perception and sleep quality in Bra-zilian older people

The study investigated the effects of a 12-week functional and resistance training program on the perception of stress and sleep quality in Brazilian older people, involving 49 participants. The Perceived Stress in older adults and the Pittsburgh Sleep Quality Index (PSQI) was used. The training protocol included a general warm-up, resistance and functional training sessions, followed by a cool-down phase, held twice a week, lasting an average of 60 minutes over the 12 weeks. Data analysis was performed using the Kolmogorov-Smirnov test, bootstrapping, t-tests and Pearson correlation, with a significance level of p < 0.05. The results showed a significant reduction in sleep disturbance and total sleep quality score after the intervention. Men initially had lower stress and better sleep quality in some aspects compared to women, while older people between 60 and 69 years old had improvements in sleep quality in certain aspects before and after the intervention, compared to older people aged 70 and over. Furthermore, a positive correlation was observed between the perception of stress and several components of sleep quality, such as subjective quality, duration and total score. In short, the study demonstrated that the exercise program had varying impacts on sleep quality, but not on the perception of stress among the older people, highlighting a significant improvement in sleep quality after the intervention. Keywords: Aging. Exercise Therapy. Sleep Quality. Psychological stress. Physical education.

HYSTERESIS OF IMPLICIT EQUATIONS IN HYPOPLASTICITY FOR SOIL MATERIALS WITH GRANULAR HARDNESS DEGRADATION

AbstractWe study a hypoplastic model for soil and granular materials stemming from geomechanical engineering which further incorporates effects of degradation of the granular hardness, therefore allowing for the description of environmental weathering. The governing system is described by a nonlinear system of transcendental-differential equations for stress and strain rate, which is investigated with respect to its long-time dynamic. Under deviatoric stress control, two different solutions of the underlying, implicit differential equations are constructed analytically. The spherical components of stress and strain rate converge asymptotically to an attractor and lead to the sparsification of material states. Whereas under cyclic loading-unloading carried out in a numerical simulation, finite ratcheting of the deviatoric strain rate is observed in the form of a square spiral.

Alternative Low-Order Triangular Elements for Thick and Thin Shell Analysis

In this paper, the authors discuss the development of six low-order triangular elements for the linear static analysis of thick and thin shell structures. The elements combine two triangular plane stress elements with three triangular thick plate bending elements to produce three-node flat shell elements with 18 degrees of freedom. The two plane stress components are based on the free formulation and the template formulation, while the three plate bending components are based on the free formulation, the deviatoric strain formulation and the hybrid-Trefftz formulation. The theoretical backgrounds and formulations of all elements are briefly discussed to provide a concise summary, while further details are given in the quoted references. The elements are used to analyze five standard shell benchmark examples, and the results obtained are compared to previous work in this area. The elements provide accurate and robust results for the benchmarks, with only small differences for the results as the meshes are refined. While all the proposed elements can be recommended for general shell analysis, the combination of the template plane stress element and the hybrid-Trefftz plate bending element generally provides the best results.

Biomechanical analysis using finite element analysis of orbital floor fractures reproduced in a realistic experimental environment with an anatomical model

Introduction: In this study, we attempted to demonstrate the actual process of orbital floor fracture visually and computationally in anatomically reconstructed structures and to investigate them using finite element analysis.Methods: A finite element model of the skull and cervical vertebrae was reconstructed from computed tomography data, and an eyeball surrounded by extraocular adipose was modeled in the orbital cavity. Three-dimensional volume mesh was generated using 173,894 of the 4-node hexahedral solid elements.Results: For the cases where the impactor hit the infraorbital foramen, buckling occurred at the orbital bone as a result of the compressive force, and the von Mises stress exceeded 150 MPa. The range of stress components included inferior orbital rim and orbital floor. For the cases where the impactor hit the eyeball first, the orbital bone experienced less stress and the range of stress components limited in orbital floor. The critical speeds for blowout fracture were 4 m/s and 6 m/s for buckling and hydraulic mechanism.Conclusion: Each mechanism has its own fracture inducing energy and its transmission process, type of force causing the fracture, and fracture pattern. It is possible to determine the mechanism of the fracture based on whether an orbital rim fracture is present.

Effect of Tip Gap Size on the Tip Flow Structure and Turbulence Generation in a Low Reynolds Number Compressor Cascade

Abstract Efficient and compact axial compressors are currently undergoing rapid development for use in microcooling systems and small-scale vehicles. Limited experimental work concentrates on the inner flow field of the compressors working at such low Reynolds numbers (Re∼104). This study examines the vortical structures and the resulting turbulence production in the transitional flow over a C4 compressor blade at a Reynolds number Re of 24,000, with a specific focus on the impact of tip clearance. The particle image velocimetry measurements reveal the tip flow structures in detail, including the tip leakage vortex (TLV) and its induced complex vortical structures. The tip secondary flow at the low Reynolds number can be divided as the tip leakage flow (TLF)/vortex and transitional boundary layer both at the end walls and the blade surfaces. The TLV propagates at the highest spanwise positions and farthest pitchwise positions at the middle tip gap size (τ/C = 3%) for the three tip gap sizes investigated. The tip flow fluctuations decrease from τ/C = 5% to τ/C = 3% and then increase from τ/C = 3% to τ/C = 1%. The spatial distribution, streamwise evolution, and individual Reynolds normal stress components contributing to the turbulent kinetic energy (TKE) are discussed. The primary contributors to the turbulence generation are examined to elucidate the flow mechanism leading to the distinct anisotropic turbulence structure in the tip region with various tip gap sizes.

Enrichment of the element free Galerkin method for cracks and notches without a priori knowledge of the analytical singularity order

We introduce a novel enrichment technique for the element free Galerkin (EFG) method to capture the infinite stress field around the crack or notch tip. The singular enrichment bases are built through a weighted residual application of the governing partial differential equation. A special mapping is used to adapt the bases with the singularities. These bases are used in a moving least squares approximation to enrich the smooth polynomial bases of the EFG within the singularity-dominated zone in an intrinsic approach. The novelty of the method is that no a priori knowledge of the analytical singularity order is required to reproduce the singular bases, but they are automatically built during the solution procedure. The diffraction technique has been used to model the discontinuity of the notch tip. The effect of various parameters, including the enriched region size, enrichment factors, nodal distribution and irregularities, has been examined. Validation of the method using available analytical solutions or finite element modeling, revealed that the proposed enrichment is quite effective and accurate for displacement and stress components.

Dynamic response of a homogeneous hygrothermoelastic slab sandwiched between elastic half-spaces

In the current study, the authors discuss a homogeneous hygrothermoelastic slab of thickness 2h sandwiched between two similar elastic half-spaces. The hygrothermoelastic slab is subjected to a mechanical source of constant magnitude. The source is applied along the interface of the slab and upper elastic half-space. The analytical expressions of the components of displacement, stresses, moisture concentration, and temperature field are obtained by the normal mode analysis technique. The analytical results are used to solve the problem numerically by taking a wood slab as a porous material. The graphical results elaborate the effect of the thickness of a slab on the physical quantities. The problem of hygrothermoelastic layers sandwiched between elastic half-spaces is a novel idea, which finds its applications in various engineering models. The results found are interesting in the context of the problem. It is observed that the thickness of a hygrothermoelastic slab affects the deformation in the medium and the values of physical quantities decrease with an increase in the thickness of the slab.

The effect of highly elongated micro-contacts on rolling contact stress field

This study presents a boundary element model to analyze contact pressure distribution and the subsurface stress field in rough rolling elements. The methodology is demonstrated by the stress field of the backup roll and work roll in the cold/temper rolling process and characterized by a parallel orientation of roughness lay with rolling direction, resulting in a cluster of highly elongated elliptical micro-contacts. The numerical outcomes revealed that the amplitude of in-plane and out-of-plane shear stress components are strongly dependent on the ellipticity of the micro-contacts. Comparison with experimental results shows that the crack plane is oriented transversal to the rolling direction and the crack initiation location corresponds to the location where the shear stress components are maximum.

Quantitative shear stress imaging in high throughput in-line Si wafer inspection

Defect inspection methods are basic steps in electronic chip manufacturing and power device production processes in the semiconductor industry. Based on stress induced photoelastic anisotropy, in-line polarized stress imaging (PSI) and quantitative determination of the local shear stress component are possible in single crystalline silicon wafers. Hereafter, such polarization stress images are compared and validated quantitatively with results of finite element simulations based on different 3D elastic models. Uniquely, high resolution at high throughputs is achieved for 300 mm wafer size in the present quantitative stress imaging study.

Enhanced susceptibility of high-strength fastener nuts to hydrogen-induced stress corrosion cracking

Abstract High-strength HV-fastener sets of dimensions M48 and M64 with property class 10.9 were employed in offshore wind turbine frameworks. The M64 were used in coupling flanges within monopiles. The M48 were employed in another offshore wind farm and encountered natural weathering. In both installations, time-delayed fractures of the nuts were observed. Owing to the presence of macroscopically visible corrosion products, hydrogen-induced stress corrosion cracking (Hi-SCC) was established as the probable cause of failure. However, a nut fracture in a properly pre-tensioned bolt assembly is atypical since the stresses in the bolt threads are higher than those in the nuts. Based on the Hi-SCC theory, the fracture should occur at the most stressed component, which is the bolt. During the root cause analysis, extensive examinations were conducted to determine the cause of the nut fractures. The focus was on investigating whether the nut material was more prone to Hi-SCC than the bolt material. The examination program included scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS) analysis of the fracture surfaces, optical microscopy of microspecimens, mechanical tests, and stress rupture tests of hydrogen-charged specimens. While the results suggest that the tested nuts comply with the requirements of the applicable standards regarding material properties, they also reveal that the nut material is, despite its lower tensile strength, significantly more susceptible to Hi-SCC than the bolt material. Therefore, a direct relationship between material susceptibility to Hi-SCC and the tensile strength, as standards and guidelines imply, is not given.

A Physics-Based Tweedie Exponential Dispersion Process Model for Metal Fatigue Crack Propagation and Prognostics

Most structural faults in metal parts can be attributed to fatigue crack propagation. The analysis and prognostics of fatigue crack propagation play essential roles in the health management of mechanical systems. Due to the impacts of different uncertainty factors, the crack propagation process exhibits significant randomness, which causes difficulties in fatigue life prediction. To improve prognostic accuracy, a physics-based Tweedie exponential dispersion process (TEDP) model is proposed via integrating Paris Law and the stochastic process. This TEDP model can capture both the crack growth mechanism and uncertainty. Compared with other existing models, the TEDP taking Wiener process, Gamma process, and inverse process as special cases is more general and flexible in modeling complex degradation paths. The probability density function of the model is derived based on saddle-joint approximation. The unknown parameters are calculated via maximum likelihood estimation. Then, the analytic expressions of the distributions of lifetime and product reliability are presented. Significant findings include that the proposed TEDP model substantially enhances predictive accuracy in lifetime estimations of mechanical systems under varying operational conditions, as demonstrated in a practical case study on fatigue crack data. This model not only provides highly accurate lifetime predictions, but also offers deep insights into the reliability assessments of mechanically stressed components.

Thermal structural damage and thermal decomposition characterization of GAP propellants with nitrate ester plasticizers

AbstractGAP and nitrate ester compounds are introduced into the solid propellant formulation as energetic binders and energetic plasticizing agents, respectively, to further enhance the energy level of solid propellants. However, under abnormal thermal conditions, various components within GAP propellants, especially nitrate ester plasticizers, can collectively result in the generation of a large number of voids within the propellant due to factors such as thermal stress and slow component decomposition. This phenomenon can impact the safety of solid rocket engines, necessitating research into their thermal decomposition processes and thermal damage structures. In this study, the thermal decomposition characteristics and gas products of GAP propellants with different nitrate ester plasticizer formulations were investigated using DSC‐TG and FT‐IR. The damage structure of GAP propellants heated under unignited conditions was studied through Micro‐CT, examining the influence of heating conditions and nitrate ester plasticizers on the thermal damage structure of GAP propellants. During heating, the thermal damage structure of GAP propellants was found to include voids generated within the GAP binder and cracks at the interface between the GAP binder and particles, with nitroglycerin as a plasticizer exacerbating the thermal damage of GAP propellants (about 2.2–2.9 times).

Additive Manufacturing of Composite Polymers: Thermomechanical FEA and Experimental Study.

This study presents a comprehensive approach for simulating the additive manufacturing process of semi-crystalline composite polymers using Fused Deposition Modeling (FDM). By combining thermomechanical Finite Element Analysis (FEA) with experimental validation, our main objective is to comprehend and model the complex behaviors of 50 wt.% carbon fiber-reinforced Polyphenylene Sulfide (CF PPS) during FDM printing. The simulations of the FDM process encompass various theoretical aspects, including heat transfer, orthotropic thermal properties, thermal dissipation mechanisms, polymer crystallization, anisotropic viscoelasticity, and material shrinkage. We utilize Abaqus user subroutines such as UMATHT for thermal orthotropic constitutive behavior, UEPACTIVATIONVOL for progressive activation of elements, and ORIENT for material orientation. Mechanical behavior is characterized using a Maxwell model for viscoelastic materials, incorporating a dual non-isothermal crystallization kinetics model within the UMAT subroutine. Our approach is validated by comparing nodal temperature distributions obtained from both the Abaqus built-in AM Modeler and our user subroutines, showing close agreement and demonstrating the effectiveness of our simulation methods. Experimental verification further confirms the accuracy of our simulation techniques. The mechanical analysis investigates residual stresses and distortions, with particular emphasis on the critical transverse in-plane stress component. This study offers valuable insights into accurately simulating thermomechanical behaviors in additive manufacturing of composite polymers.

An integrated geomechanical and petrophysical multiparameter approach for gas reservoir evaluation

AbstractIntegrating petrophysical and geomechanical parameters is an efficient approach to evaluating shale gas reservoir potential. The high cost of corings and their limited number, coupled with time-intensive investigation, led researchers to use this alternative combination approach. In the Jiaoshiba area, from single-pilot well core data and log measurements, petrophysical and geomechanical parameters such as shale volume, total organic carbon, gas content, as well as pore pressure, stress components, and mineral brittleness were first estimated using established methods. In the second phase, based on logging curves, the reservoir electro-facies (EF) classification was performed using the unsupervised multi-resolution graph-based clustering method on a series of twenty wells, identifying five EF with different intrinsic characteristics. Unsupervised analyses were developed using the multilayer artificial neural network while incorporating the K-nearest neighbors and graphical classification algorithms. The results from the first and second phases indicate reservoir richness in organic matter, with the best reservoir exhibited by EF2 and EF3. In addition, effective stress components (SV, SH, and Sh) evaluation shows a normal stress regime with hydraulic fracture systems perpendicular to the minimum horizontal stress at each measured depth of the reservoir (Sv > SH > Sh). This research workflow can efficiently evaluate shale reservoirs with a realistic approach for identifying favorable fracturing positions while reducing errors due to human interference.

Analysis of the crack problem under unified generalized thermoelasticity

The present article aims to manifest a two-dimensional dynamical problem based on a unified way of dual-phase lags generalized thermoelasticity. The problem deals with finite mode-I crack due to tensile force in an unbounded linear thermoelastic space under a specified stress distribution and temperature. Here the governing equations are constructed for a homogenous and isotropic medium. Boundary conditions convert the problem into four dual integral equations, whose solution corresponds to the solution of Fredholm’s integral equation of first order. The integral transform methods are applied to obtain the solution to a problem. An inversion of the Laplace transform using the Bellman method is applied to determine the numerical value of the components of temperature, stress, and displacement for copper material, which are interpreted graphically. Moreover, the stress intensity factor (SIF) near the crack center is calculated and shown graphically.

High-speed photoelastic tomography for axisymmetric stress fields in a soft material: Temporal evolution of all stress components

This study presents a novel approach for reconstructing all stress components of the dynamic axisymmetric fields of a soft material using photoelastic tomography (PT) and a high-speed polarization camera. This study focuses on static and dynamic Hertzian contact as an example of transient stress field reconstructions. For the static Hertzian contact (a solid sphere pressed against a gel block), all stress components in the urethane gel, which has an elastic modulus of 47.4 kPa, were reconstructed by PT using the measured photoelastic parameters. The results were compared with theoretical solutions and showed good agreement. For the dynamic Hertzian contact (a sphere impacting gel), a high-speed polarization camera was used to reconstruct the transient stress field within the gel. PT was used to quantitatively measure the shear and axial stress waves and showed different propagation speeds on the substrate. The technique allowed the simultaneous measurement of stress fields ranging from O(10−1) to O(101) kPa during large deformations, demonstrating its accuracy in capturing rapidly changing stress tensor components in dynamic scenarios. The scaling laws of the calculated impact force agreed with theoretical predictions, validating the accuracy of PT for measuring dynamic axisymmetric stress fields in soft materials.

A machine learning constitutive model for plasticity and strain hardening of polycrystalline metals based on data from micromechanical simulations

Machine learning (ML) methods have emerged as promising tools for generating constitutive models directly from mechanical data. Constitutive models are fundamental in describing and predicting the mechanical behavior of materials under arbitrary loading conditions. In recent approaches, the yield function, central to constitutive models, has been formulated in a data-oriented manner using ML. Many ML approaches have primarily focused on initial yielding, and the effect of strain hardening has not been widely considered. However, taking strain hardening into account is crucial for accurately describing the deformation behavior of polycrystalline metals. To address this problem, the present study introduces an ML-based yield function formulated as a support vector classification model, which encompasses strain hardening. This function was trained using a 12-dimensional feature vector that includes stress and plastic strain components resulting from crystal plasticity finite element method (CPFEM) simulations on a 3-dimensional RVE with 343 grains with a random crystallographic texture. These simulations were carried out to mimic multi-axial mechanical testing of the polycrystal under proportional loading in 300 different directions, which were selected to ensure proper coverage of the full stress space. The training data were directly taken from the stress–strain results obtained for the 300 multi-axial load cases. It is shown that the ML yield function trained on these data describes not only the initial yield behavior but also the flow stresses in the plastic regime with a very high accuracy and robustness. The workflow introduced in this work to generate synthetic mechanical data based on realistic CPFEM simulations and to train an ML yield function, including strain hardening, will open new possibilities in microstructure-sensitive materials modeling and thus pave the way for obtaining digital material twins.

Endoplasmic Reticulum Stress and Unfolded Protein Response in Vernal Keratoconjunctivitis.

Vernal keratoconjunctivitis (VKC) is an ocular allergic disease characterized by a type 2 inflammation, tissue remodeling, and low quality of life for the affected patients. We investigated the involvement of endoplasmic reticulum (ER) stress and unfolded protein response in VKC. Conjunctival imprints from VKC patients and normal subjects (CTs) were collected, and RNA was isolated, reverse transcribed, and analyzed with the Affymetrix microarray. Differentially expressed genes between VKC patients and CTs were evaluated. Genes related to ER stress, apoptosis, and autophagy were further considered. VKC and CT conjunctival biopsies were analyzed by immunohistochemistry (IHC) with specific antibodies against unfolded protein response (UPR), apoptosis, and inflammation. Conjunctival fibroblast and epithelial cell cultures were exposed to the conditioned medium of activated U937 monocytes and analyzed by quantitative PCR for the expression of UPR, apoptosis, autophagy, and inflammatory markers. ER chaperones HSPA5 (GRP78/BiP) and HYOU1 (GRP170) were upregulated in VKC patients compared to CTs. Genes encoding for ER transmembrane proteins, PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), ER-associated degradation (ERAD), and autophagy were upregulated, but not those related to apoptosis. Increased positive reactivity of BiP and ATF6 and unchanged expression of apoptosis markers were confirmed by IHC. Cell cultures in stress conditions showed an overexpression of UPR, proinflammatory, apoptosis, and autophagy markers. A significant overexpression of genes encoding for ER stress, UPR, and pro-inflammatory pathway components was reported for VKC. Even though these pathways may lead to ER homeostasis, apoptosis, or inflammation, ER stress in VKC may predominantly contribute to promote inflammation.

A methodological approach to evaluate the dynamic response of internal septa and pillars in soft rock underground cavities

AbstractA large amount of underground cavities nowadays exists throughout the Apulian region (south-eastern Italy) as a result of mining processes of soft calcarenite, which frequently followed the “room and pillar” technique. In these cave systems, pillars and septa are critical structures, whose failure can lead to a significant increment of the sinkhole hazard. The behaviour of these rocky structures in the dynamic field is poorly studied in literature, and the present study aims to investigate their dynamic stability, according to regional seismicity data. For this purpose, ideal pillar geometries were considered, for which the evolution of the stress–strain field under dynamic inputs was observed in both 2D and 3- configurations by means of parametrical finite element analysis. For shallow cavities, slender septa were found to be the most affected by the influence of seismic loading. For deep cavities, dynamic instability is observed only for rather squat septa, with the cavity width also influencing the dynamic behaviour. To quantitatively assess the septum stability, a stability index was also proposed in 2D models. Moreover, three-dimensional analyses showed a stabilizing effect in the pillar exerted by the stress component perpendicular to the earthquake.